Don't re-create singletons

Don't re-create singletons

Summary:
This forbids singleton creation/re-creation after destroyInstances() was called.
Only after reset() is called singletons can be created again.

registrationComplete() behavior is also slightly change. We disallow singleton registration after registrationComplete() is called even in Relaxed mode. Strict mode now only controlls whether singletons can be constructed before registation is complete.

Test Plan: unit test

Reviewed By: chip@fb.com

Subscribers: hphp-diffs@, ps, trunkagent, njormrod, mshneer, lins

FB internal diff: D1605136